Tech Tips: AC Motor Basics

Understanding the characteristics of AC motors allows engineers select the motor best suited to their application.

Video Transcript: Groschopp Tech Tips – AC Motor Basics

Hello!  This is Arlo with a Groschopp Tech Tip.  Today we will be discussing AC Induction Motors basics, with a brief overview of single phase and 3-phase motors.

AC motors are the most common motor used in applications because they use AC voltage (available at every outlet), run quietly, have a long life, and are economical.

All AC motors have the same basic components, a stator and rotor. The stator is the stationary coils in the motor where the current is produced to create the magnetic field. This magnetic field induces a current in the rotor bars causing the rotor to rotate.

In a 3 phase motor, the current in the stator sets up a rotating magnetic field. The magnetic field rotates due to the 120° phase offset in each phase of the power supply. This rotating magnetic field induces a current in the bars of the rotor. The current in the rotor sets up its own magnetic field. The interaction between the stator and rotor magnetic fields causes the rotor to rotate. An important thing to remember with 3 phase motors is since they run off of 3 phases that are offset from each other, they are self-starting.

Single phase motors work on the same principle as 3 phase motors except they are only run off one phase. A single phase sets up an oscillating magnetic field that goes back and forth rather than a rotating magnetic field. This results in a true single phase motor having zero starting torque. However, once the rotor begins to rotate it will continue to rotate as a result of the oscillating magnetic field in the stator. There are many ways to start single phase motors. Most involve producing a second phase to help produce a rotating magnetic field in the stator. Some common types are the shaded pole motor, the split phase motor, the permanent split capacitor motor or single value capacitor motor), and the two value capacitor motor.

Since we are focusing on induction or asynchronous AC motors, we need to briefly discuss slip.    Slip is the difference between the synchronous speed and the actual speed of the motor. Induction motors require slip to induce current in the rotor, and the amount of slip changes as the load on the motor changes. In order to change the speed of an induction motor, the frequency must be changed. This is accomplished by a motor control, and the most common is a Variable Frequency Drive (VFD). Without a control, the motor speed is fixed by the equation.  Determining factors are input frequency and poles of the AC Motor.

Two key characteristics of AC motors to consider for any application are running speed and starting torque. First, let’s look at running speed. This is dependent on the power supply frequency, the number of motor poles and the amount of slip of the rotor.  For line driven motors the pole count will be defined by the rotor speed less some slip due to load.  The performance curve of the motor will show what the motor’s maximum available starting torque is.

The second characteristic of AC motors it starting torque.  It is the chief limitation of the AC motor. Remember, if your application requires the motor to start with a load on it, consult your motor manufacturer to ensure the motor has enough torque to start at load and to ensure the correct motor type is specified for your application.

In comparison to single phase motors, 3 phase motors have a higher power density, a higher starting torque, and are more efficient than single phase motors. They start on their own, eliminating the need for a starting winding or capacitor. The same speed calculation applies to 3 phase motors as single phase, so a VFD is required to change the motor speed. Also, when a 3 phase power source is unavailable, controls are available to convert single phase power to 3 phase, making the 3 phase motor more versatile.

AC motors work great for a number of different applications, such as pumps, conveyors, and commercial products. Remember the key characteristics when considering an AC motor, and consult your motor manufacturer with your application specifications to ensure you are getting the correct motor for your application.

This has been a Groschopp Tech Tip.  For more information about any of our products or to view other Tech Tips please visit us online at www.groschopp.com.

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